In a large mixing bowl, combine cream cheese, sour cream, and mayonnaise until smooth.
Stir in spinach, artichoke hearts, Parmesan cheese, mozzarella cheese, garlic, salt, black pepper, and red pepper flakes (if using).
Transfer mixture to a greased 9-inch baking dish and spread evenly.
Bake for 20–25 minutes, or until bubbly and lightly golden on top.
Let cool for 5 minutes before serving. Serve warm with tortilla chips, pita bread, or crackers.
Notes
For a lighter version, substitute reduced-fat cream cheese, sour cream, and mozzarella. Can be made ahead and refrigerated; add 5–10 minutes to baking time if cold.